You don't even mention the really insecure inputs to printers - many older printers have infrared links, new ones have USB ports, bluetooth, wi-fi, etc. most of which don't work exactly as advertised. Did you ever try to get a printer to print a file vai ir link from a handheld device? Takes a lot of fussing around.

Anything an ordinary person can walk up to and fiddle with is inherently insecure...